The present invention concerns a device for continuously processing material webs with ultrasound and a sonotrode therefor.
Ultrasound is a mechanical oscillation above the audible limit. The frequency range begins at about 20 kHz and extends to frequencies of 1 GHz. Such ultrasonic frequencies are frequently generated by means of piezoelectric sound transducers (converters) from electrical energy. That mechanical oscillation energy is applied to the workpiece or the material to be processed by way of the sonotrode which is connected to the converter, possibly by way of an amplitude transformation member (also referred to as a booster).
The surface of the sonotrode intended to come into contact with the material to be processed is also referred to as the sealing surface. The oscillating unit thus comprises generator, converter, optionally the amplitude transformation member and the sonotrode. The converter and the sonotrode are formed in one piece for many use situations.
To effectively transmit the ultrasonic oscillation by means of the ultrasonic oscillating unit it is necessary to cause the ultrasonic oscillating unit to assume a resonance condition. In dependence on the structure of the ultrasonic oscillating unit it has a multiplicity of natural frequencies. It is only when the converter generates a natural frequency of the ultrasonic oscillating unit that resonant oscillation of the unit occurs. Therefore the converter and ultrasonic oscillating unit must be tuned to each other.
When processing materials by means of ultrasound generally the material to be processed is positioned between the sonotrode and a counterpart tool (not belonging to the oscillating structure), which is also referred to as the anvil. The sonotrode in contact with the material to be processed then transmits the ultrasonic energy to the material to be processed, which is for example thereby welded or severed.
Packaging technology frequently uses so-called horizontal or vertical tubular bag packaging machines. Therein a web of material is drawn from the roll, shaped to form a tube and the longitudinal edges are sealed. Thereafter the bag is filled, sealed in the transverse direction and finally cut off. Therefore welding of the transverse seam produces both the head seam of the filled bag and also the bottom seam of the next bag.
Ultrasonic sonotrodes are frequently used to produce the longitudinal seam in the tubular bag packaging machines. In that case the webs of material to be joined are moved continuously between the sonotrode and the counterpart tool.
Mostly hot sealing devices are used at the present time for producing the transverse sealing seam. Recently however ultrasonic processing devices are also used individually for producing the transverse sealing seam. These however are generally discontinuously operating sonotrodes, that is to say the sonotrode does not rotate but the substantially rectangular sealing surface is periodically brought into contact with the material web to produce the transverse sealing seam.
U.S. Pat. No. 6,574,944 already proposed rotational sonotrodes for producing a transverse sealing seam by means of ultrasound. In the embodiment shown therein, four separate sonotrodes are mounted on a parallelepipedic drive shaft. When the drive shaft is rotated the four sonotrodes are successively brought into contact with the material web and produce a respective transverse sealing seam.
The embodiment with four sonotrodes mounted on a shaft however is complicated and expensive to produce. In addition, it is only possible with a high energy consumption to cause the sealing surfaces of the four sonotrodes to oscillate by way of the drive shaft. Therefore the structure shown in U.S. Pat. No. 6,574,944 has not been successful in practice.